Listeria monocytogenes (Lm) is important an important pathogen to study because: 1) it causes life threatening food borne infections of significant public health concern, 2) it is a genetically tractable organism with a unique intracellular lifestyle used as a tool for understanding the cell biology of mammalian cells, and 3 systemic (i.v.) listeriosis is a highly reproducible infection frequently used by immunologists to study cell- mediated immune responses. Due to the lack of a small animal model that closely mimics human disease, we still know very little about oral transmission of Lm, or why the innate susceptibility to developing severe gastroenteritis, sepsis, and meningoencephalitis varies among individuals. To address this knowledge gap, we recently developed a new model of oral listeriosis using mice fed Lm-contaminated food. This natural feeding model revealed clear differences in the ability of Lm to colonize the gut and spread systemically in susceptible (BALB) vs. resistant (B6) mice. Thus, for the first time, we can now study how gut-adapted Lm that survive digestive processes in the stomach are able to colonize the intestinal mucosa and serve as a nidus for continual seeding of peripheral tissues in susceptible mice unable to quickly clear the gut infection. Since the vast majority of patients hospitalized with listeriosis can be considered immune compromised in some way, it has long been thought that protective immune responses were critical for limiting the infection to a mild gastroenteritis in resistant individuas. Our central hypothesis predicts that one such innate immune mechanism is the rapid secretion of IFN? by a subset of memory phenotype CD8+ T cells. We postulate that T cell-derived IFN? can rapidly initiate clearance mechanisms in B6 mice and that the delay in activation of CD8+ T cells in BALB mice allows Lm to replicate exponentially and spread to systemic tissues. This application has three specific aims: (1) to identify the cell types in the colon that support Lm growth and thus, serve as the targets of protective innate immune responses; (2) to identify the specific subsets of CD8+ T cells that rapidly secrete IFN? after ingestion of Lm and show that this T cell-derived IFN? can rapidly limit the intracellular growth of Lm in the colon; (3) to idenify IFN?-dependent innate immune mechanisms that promote rapid clearance of Lm in peripheral tissues after dissemination from the gut. A key strategy in this application will be the use of a unique adoptive transfer system wherein T cells from a responsive mouse are injected into a MHC-matched non-responsive mouse. This powerful approach will allow us to specifically isolate the function of IFN? rapidly produced by CD8+ T cells while leaving all other innate immune mechanisms intact. The expected outcomes of the proposed studies are: [1] significant new insights into the pathogenic life style of food borne Lm, and [2] a better understanding of the innate immune mechanisms that determine host resistance/susceptibility to intracellular bacterial pathogens.